Because of high paint deposition efficiency and satisfactory finish quality, rotary atomizing head type coating machines have been widely used for coating vehicle bodies, furniture, electric appliances and the like. A rotary atomizing head type coating machine is composed of a tubular housing for accommodating a motor, an air motor accommodated within a motor compartment of the housing to drive a rotational shaft by a turbine, a bell- or cup-shaped rotary atomizing head mounted on a fore end portion of the rotational shaft of the air motor at a position on the front side of the housing, and a paint passage for paint supply to the rotary atomizing head (e.g., as disclosed in Japanese Patent Laid-Open No. S60-14959 and H8-1046).
The housing of the rotary atomizing head type coating machine is provided with a turbine air passage for turbine air which drives the turbine of the air motor, and an exhaust air passage for discharging exhaust turbine air to the outside from the turbine of the air motor. In this instance, the turbine air which drives the air motor is clean and sufficiently dry air and supplied under predetermined pressure and at a predetermined flow rate.
Further, certain rotary atomizing head type coating machines are provided with a high voltage generator to apply a high voltage to a paint which is supplied to the rotary atomizing head. Paint particles which are charged with a high voltage are urged to fly toward a work along an electric line of force and efficiently deposited on the work.
In the case of the prior-art rotary atomizing head type coating machines mentioned above, sufficiently dry air is supplied to an air motor as turbine air. However, these days coating machines are required to raise the turbine speed in the range of 3,000 to 100,000 r.p.m. in order to spray even a highly viscous paint from a rotary atomizing head in finely atomized particles and to atomize a paint which is supplied at a high flow rate. Therefore, it becomes necessary to supply an air motor with turbine air under an increased pressure of 3-6 kg/cm2 and at a higher flow rate of 100-600 NL/min. Besides, turbine air is at a high temperature.
In a case where the turbine air pressure is increased in this way, an abrupt drop in temperature occurs due to adiabatic expansion when turbine air of high pressure and high temperature is introduced into a turbine chamber, and exhaust turbine air which has been used for driving the turbine comes out at a low temperature. Therefore, the air motor and the housing and other components in the surroundings are constantly cooled by exhaust turbine air. In addition, not only an exhaust air passage which exhaust turbine air flows through but also the rear portion of the housing and other components around the exhaust air passage are cooled by the flow of exhaust turbine air.
In this connection, a coating operation is carried out in a coating booth which is kept at suitable temperature and humidity from the standpoint of giving a good finish to coatings. For instance, in the case of a coating booth which is used for coating vehicle bodies, the booth temperature and humidity are maintained at 20° C.-25° C. and 70%-90%, respectively. Therefore, if the housing is cooled by cold exhaust air, moisture condensation or sweating is very likely to occur on housing surfaces in a coating booth of high temperature and humidity.
The moisture condensation on housing surfaces gives rise to a problem that a high voltage to be applied to a paint is leaked to the earth ground, making an electrostatic coating operation infeasible. Further, if the housing is electrically connected to the earth ground by moisture condensation, paint particles which are charged with a high voltage are urged to fly toward and deposit on a surface of the housing accelerating degradations in electrical insulation properties of housing surfaces.
Furthermore, with a progress of moisture condensation on housing surfaces, water droplets are formed by condensed water and, if the coating machine is operated in this state, the water droplets are spattered to deposit on coated surfaces. In such a case, the quality of coating is degraded to a considerable degree even if the deposited water droplets are small in particle size or in amount.